The present invention relates to an image forming apparatus based on electrophotographic technology.
In the image forming apparatus based on the electrophotographic technology, a large amount of coated paper in addition to normal paper has come into widespread use in recent years, as a result of growing popularity of a color image forming apparatus. The normal paper is a transfer paper whose surface is not provided with coating, and is commonly employed in extensive applications. In the meantime, the coated paper is a transfer paper whose surface is coated with such a pigment as activated clay and is made smooth to improve the finish subsequent to image formation. The coated paper is further classified into calendared paper and other types, depending on the type of the coating medium to be used, and the degree of smoothness on the surface (Patent Document 1, that is, Japanese Unexamined Patent Application Publication No. 2004-284804).
On the other hand, in an image forming apparatus based on electrophotographic technology, a developer bearing member for bearing the developer (toner in the case of a one-component development, and toner and carrier in the case of a two-component development) is moved relative to the photoreceptor with an electrostatic latent image formed thereon, whereby the electrostatic latent image on the photo-receptor is developed. In this case, to ensure that the background fog (toner adhered to the background where toner should not adhere) does not occur, a potential difference is provided between the surface potential of the photoreceptor background portion and the bias potential of the developer bearing member (hereinafter referred to simply as “development bias” in some cases). This potential difference is called “fog margin”.
However, despite adequate setting of the fog margin, the background fog (hereinafter referred to as “fog”) will be deteriorated by a change with passage of time in printing a large number of sheets. This is because the developer is deteriorated by stress such as stirring, and therefore increases the amount of fog-causing poorly charged toner including insufficiently-charged toner, uncharged toner or oppositely charged toner.
One of the efforts to solve this problem is disclosed in the Patent Document 2 (Japanese Unexamined Patent Application Publication No. 05-224512) wherein toner density of toner fog is detected by a toner sensor while the development bias is changed, and the characteristic curve of toner density with respect to development bias is obtained. If the development bias capable of outputting the toner density when toner is no adhered is higher than a reference level, copying operation is carried out by increasing development bias by a predetermined amount, thereby solving the problem caused by a rise in fogging level. That is, there is described the image forming apparatus to cope with the deterioration of the fogging with the passage of time by changing the fog margin.
However, in the image forming apparatus disclosed in the Japanese Patent Application Publication No. 05-224512, the setting of fog margin is changed to eliminate the possibility of causing a fog at all times. Thus, the poorly charged toner is continuously stored in a development tank without being consumed as fog toner. With the process of time, the poorly charged toner cannot be dealt by the adjustment of fog margin, and fog deterioration occurs in a short time, as a result.
In an image forming apparatus using both normal paper and coated paper, toner transfer efficiency for the coated paper is higher than that for the normal paper, even if the amount of fog toner deposited on the photoreceptor is the same. Even if no fog occurs to the normal paper, fog does occur to the coated paper. Thus, this requires the fog margin to be set greater than that in the image forming apparatus designed for the use of normal paper alone. As a result, a greater amount of charged toner is accumulated in the development tank and fog deterioration occurs in a shorter time. Such problems have been left unsolved in the conventional art.
The object of the present invention is to solve the aforementioned problems and to provide an image forming apparatus using both normal paper and coated paper wherein high-quality image of less conspicuous fog is provided for both the normal paper and coated paper, and fog deterioration is minimized for a long period of time. This object can be achieved by the following structure: